1. Technical Field
The present invention relates to a spark plug for an internal combustion engine having a noble metal chip joined only to a ground electrode, and which is adapted to generate spark discharge between the same and a center electrode facing the same, as well as to an internal combustion engine equipped with the spark plug.
2. Description of the Related Art
Conventionally, a spark plug is used for ignition of an internal combustion engine. A spark plug generally includes an insulator having an axial hole axially extending therethrough, and holding a center electrode in a front end portion of the axial hole; a metallic shell surrounding and holding a trunk portion of the insulator; and a ground electrode having a first end welded to a front end of the metallic shell and a second end facing a front end of the center electrode, thereby forming a spark discharge gap therebetween.
In recent years, an increase in temperature in a combustion chamber associated with an improvement in engine output has led to a demand for spark plugs having high-temperature-corrosion resistance and oxidation resistance. In order to reduce electrode consumption which accompanies spark discharge at the time of ignition, there is also a need for resistance to spark-induced consumption. Under these circumstances, a noble-metal chip for enhancing resistance to spark-induced consumption is provided on opposing faces of each of a center electrode and a ground electrode (refer to, for example, Patent Document 1).
In a case where such spark plugs are used in an internal combustion engine, generally, the spark plugs are energized for spark discharge such that the center electrodes assume a negative polarity, whereas the ground electrodes assume a positive polarity. However, in some internal combustion engines, in order to enhance the efficiency of energization, the mounted spark plugs are energized so as to alternate positive polarity and negative polarity from cylinder to cylinder. As compared with an electrode of negative polarity, a positive polarity electrode exhibits greater consumption accompanying spark discharge. Accordingly, such an internal combustion engine employs two kinds of spark plugs; i.e., a spark plug in which resistance to spark-induced consumption of a center electrode is enhanced, and a spark plug in which resistance to spark-induced consumption of a ground electrode is enhanced. These spark plugs are selectively mounted on respective cylinders so as to be compatible with the polarity of the cylinders.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2003-197347
3. Problems to be Solved by the Invention
However, for example, replacement of spark plugs has involved potential attachment of a spark plug of the wrong polarity due to human error or the like. Attachment of a spark plug of the wrong polarity to a cylinder involves risk of abnormal consumption of an electrode. This problem can be effectively solved by using a spark plug in which a noble-metal chip is provided on each of a center electrode and a ground electrode so as to enhance resistance to spark-induced consumption, such as the spark plug described in Patent Document 1. This allows use of the same spark plug with a cylinder of either polarity. However, this is disadvantageous in that a noble-metal chip is provided even on an electrode whose consumption is not intensive due to its polarity, with the result that a more expensive spark plug is unnecessarily used.